Method for producing a metal-ceramic substrate with at least one via

ABSTRACT

A method for producing a metal-ceramic substrate with at least one electrically conductive via, in which one metal layer, respectively, is attached in a planar manner to a ceramic plate or a ceramic layer to each of two opposing surface sides of the ceramic layer is provided. The method includes introducing a metal-containing, powdery and/or liquid substance into a hole in the ceramic layer delimiting the via prior to the attachment of both metal layers, or subsequent to the attachment of one of the two metal layers to form an assembly. Prior to the attachment of the other one of the two metal layers, and the assembly is subjected to a high-temperature step above 500° C. in which the metal-containing substance wets the ceramic layer at least partially with a wetting angle of less than 90°.

TECHNICAL FIELD

The present invention relates to a method for producing a metal-ceramicsubstrate, in particular a ceramic substrate metallized on both sides,with at least one electrically conductive.

BACKGROUND

Such metal-ceramic substrates are preferably used in the field of powersemiconductor modules. In the process, a ceramic substrate, e.g.aluminum-oxide ceramics, is provided on its top and bottom sides with ametallization, wherein, generally, at least one metallized side laterhas a circuit structure that is generated by means of etching processes,for example. The known method for producing these ceramic substratesmetallized on both sides is performed by means of eutectic bonding, andis generally referred to as the direct bonding process.

A basic description of a method for producing metallized ceramicsubstrates by bonding processes is available from EP 0 085 914 A2, forexample. In the context of the direct copper bonding process (DCBprocess), for example, a copper sheet is first oxidized so as to resultin a substantially uniform copper oxide layer. The resulting coppersheet is then positioned on a surface side of a ceramic substrate, andthe composite of the ceramic substrate and the copper sheet is heated toa bonding temperature between about 1025° C. and 1083° C., which resultsin the formation of a metallized ceramic substrate. Finally, themetallized ceramic substrate is cooled off.

Further, EP 1 403 229 B1 describes a method for producing analuminum-ceramic composite substrate in which, instead of copper, analuminum plate is attached to at least one surface side of an aluminumoxide or aluminum nitride ceramic substrate.

The production of ceramic substrates metallized on both sides can eithertake place via a two-stage bonding process, in which, first, a firstsurface side of the ceramic substrate is metallized, and then a secondsurface side of the ceramic substrate opposite from the first surfaceside is metallized, or alternatively via a single-stage bonding processin which both surface sides of the ceramic substrate are metallizedsimultaneously.

For example, DE 10 2010 023 637 B4 describes a method for producingceramic substrates, which are metallized on both sides, in a singleprocess step, wherein a first metal layer, a ceramic layer and a secondmetal layer are successively positioned on a carrier or a bonding aid inthe indicated order, and then the two metal layers are simultaneouslybonded to the ceramics in a high-temperature step in accordance with theDCB method to form a metal-ceramic substrate metallized on both sides.

Furthermore, a method for applying a metallic layer onto a layer of aceramic material is known from DE 10 2010 025 311 A1, in which ametallic sheet or a metallic plate is bonded to the layer of the ceramicmaterial by melting an intermediate layer of a eutectic mixture. Inorder to form the intermediate layer, a mixture of the metallicmaterial, an oxide of the metallic material and aluminum oxide as aceramic filler is introduced between the metallic sheet or the metallicplate and the ceramic layer. The coefficient of thermal expansion of theintermediate layer is adjusted by the content of the ceramic material inthe mixture, so that fewer mechanical stresses arise during thermalcycles.

Vias, which establish electrically conductive connections between, forexample, conductor paths, contact surfaces etc. from two different sidesof a substrate, are frequently required in electronic circuitengineering.

A method for producing a metal-ceramic substrate, which consists of aceramic substrate and two metal layers each provided on one surface sideof the ceramic layer, and which has at least one via at which the metallayers are electrically connected to each other in the area of anopening in the ceramic layer through a bridge of metal, is described inEP 0 627 875 A1, for example. Into the opening in the ceramic layerdelimiting the via, a metal body filling the opening completely oralmost completely is inserted after the ceramic layer has already beenbonded, on one surface side, to a metal layer in accordance with the DCBmethod. After the insertion of the metal body into the opening, thesecond metal layer is then attached, in a further process step, to theas yet unoccupied surface side of the ceramic layer in accordance withthe DCB method, with the electrically conductive via beingsimultaneously produced by the metal bridge. Circular-blank-shaped,cuboid or spherical configurations are proposed for the shape of themetal body. In order to produce a reliable via, the proposed methodrequires the metal body to have exactly the thickness of the ceramiclayer, so that after insertion into the opening, it ends flush with thesurface side onto which the second metal layer is applied, or to have asize that is slightly larger than the thickness of the ceramic layer,wherein the metal body in that case must be pressed flat with a punchprior to the application of the second metal layer.

Furthermore, DE 41 03 294 A1 discloses a method for producingelectrically conductive vias in ceramic substrates to whose surfaceslayers of metal are attached by means of the DCB method, wherein thehole in the ceramic layer delimiting the via is completely filled with apowder containing the metal. The assembly of the ceramic layer, thepowder-filled hole and the metal layers disposed at the surfaces of theceramic layer is subjected to a single high-temperature step accordingto the DCB method, in which the capillary action of the metal powderdraws the metal situated above the hole into it, so that the metal, inthe case of very small hole diameters, fills the hole and, in the caseof lager hole diameters, covers the hole walls in a firmly adheringmanner.

Yet another method for producing a metal-ceramic substrate, in which afirst metallization is applied to a first surface side of a ceramiclayer in the form of a copper sheet using the DCB method, and a secondmetallization is applied to the other surface side of the ceramic layerusing a paste containing the metal of this second metallization bythick-film or thin-film technology, wherein the ceramic layer has atleast one opening and, during the application of the paste, it is alsoapplied in the opening and to a surface of the first metallizationexposed in the opening, is described in EP 0 862 209 A2. The pasteapplied to the second surface side of the ceramic layer is fired under anitrogen atmosphere at a temperature that is very much lower incomparison to the temperature of the DCB method, in order to form thesecond metallization.

SUMMARY

Against this background, the present invention is based on the object ofpresenting a method for producing a metal-ceramic substrate with atleast one electrically conductive via, which is improved with respect tothe quality of the electrically conductive via, and thus of the entiremetal-ceramic substrate. In the process, the method is supposed toensure the formation of the electrical conductivity of the via andenable mass-producible vias for ceramic substrates.

It must be noted that the features cited individually in the followingdescription can be combined with each other in any technologicallymeaningful manner and represent other embodiments of the invention. Thedescription, in particular in connection with the figures, additionallycharacterizes and specifies the invention.

According to the invention, in a method for producing a metal-ceramicsubstrate with at least one electrically conductive via, one metallayer, respectively, is attached in a planar manner to a ceramic plateor a ceramic layer to each of two opposing surface sides of the ceramiclayer, wherein a metal-containing, powdery and/or liquid substance isintroduced into a hole in the ceramic layer delimiting the via prior tothe attachment of both metal layers, or subsequent to the attachment ofone of the two metal layers and prior to the attachment of the other oneof the two metal layers, and this assembly is then subjected to ahigh-temperature step above 500° C. in which the metal-containingsubstance wets the ceramic layer at least partially with a wetting angleof less than 90°.

In a manner known per se, wetting or contact angle refers to the anglethat a drop of liquid (in this case the metal-containing substance inthe high-temperature step) forms on the surface of a solid (in this casethe ceramic layer) with respect to this surface. Partial wettinggenerally refers to the wetting angle being less than 90°, i.e., thedrop of liquid forms a round dome on the surface of the ceramic layerand does not roll off. “At least partially” includes a complete wettingin which the liquid spreads on the surface of the ceramic layer in theform of a flat disk and a macroscopic contact angle is no longerprovided.

In other words, in the ceramic substrate, which is metallized on bothsides in the finished state, the metal-containing substance isintroduced into the hole delimiting the at least one via prior toattaching at least one of the two or both metal layers to the opposingsubstrate surfaces, and is then formed into a via electricallyconnecting the two substrate surfaces with each other, wherein theproperty of the metal-containing substance of at least partially wettingthe ceramic layer contributes to the metal contained in the substanceforming a bond with the ceramic layer.

Since at most one of the two metal layers is attached to one of the twosubstrate surfaces in said high-temperature step, a gas released fromthe metal-containing substance during the high-temperature step canescape unimpededly from the hole of the via. Thus, the formation ofvoids between two metal layers already applied to the substrate surfacesin the via, which affect the electrical conductivity, are reliablyavoided. According to the invention, the two metal layers, or the onemetal layer that is not yet attached, are/is attached to thecorresponding substrate surfaces or the corresponding substrate surfaceof the ceramic substrate only after the via has been produced in thehigh-temperature step, e.g. by means of a conventional DCB method.Mass-producible vias in the ceramic substrate finally metallized on bothsides can be ensured by the method according to the invention.

The hole delimiting the via may have a diameter between approximately 50μm and approximately 2000 μm. The hole can be introduced into theceramic layer or ceramic plate both by lasering after sintering theceramics, or by punching the green compact prior to sintering.

According to a preferred advantageous embodiment of the invention, theone of the two metal layers is attached to one of the two surface sidesof the ceramic layer, for example by the DCB method known per se, priorto the introduction of the metal-containing substance into the holedelimiting the via. Thus, the hole is already closed on one side by thefirst metal layer prior to the metal-containing substance beingintroduced into the hole, so that the metal-containing substance can beretained in the hole more easily after the introduction and a higherdegree of filling of the hole with the metal-containing substance can beachieved. Preferably, the degree of filling of the hole is at leastapproximately 30% of the volume of the hole. A higher degree of fillingimproves the formation of the electrical conduction within the hole andthus between the two surface sides of the ceramic layer connected toeach other through the hole.

According to another advantageous embodiment of the invention, themetal-containing substance is a powder mixture composed of copper (Cu)and additionally at least one element from the group consisting ofcopper(I) oxide (Cu₂O), copper(II) oxide (CuO) and copper(II) hydroxide(Cu(OH)₂), wherein the content of copper in the powder mixture isbetween 0% and approximately 95% and the at least one element from theaforementioned group forms the rest up to 100%.

Another advantageous embodiment of the invention provides that the grainsizes of the powder mixture are at most 90% of the diameter of the holedelimiting the via. This ensures a sufficient fillability of the hole.

For example, the powder mixture can be introduced into the hole or holesdelimiting the via by shaking. This process can be carried out in aplanar manner on the entire ceramic layer or plate, so that a largenumber of electrically conductive vias can be prepared simultaneously.

Alternatively, the powder mixture, prior to the introduction into thehole delimiting the via, can also be mixed with a carrier material toform a viscous paste, particularly preferably with a liquid thatevaporates in a residue-free manner up to a temperature of approximately400° C., such as water or an alcohol. Then, the paste can be introducedinto the hole or holes in the ceramic layer by means of, for example,stencil printing, screen printing or dispenser. This process can becarried out in a planar manner on the entire ceramic layer or plate, sothat a large number of electrically conductive vias can be preparedsimultaneously.

The carrier material or carrier liquid can evaporate already at roomtemperature. In order to shorten the production times of themetal-ceramic substrate, another advantageous embodiment of theinvention provides that the carrier liquid is evaporated in anadditional temperature step up to a maximum of 400° C. after theintroduction of the paste and prior to carrying out the high-temperaturestep.

According to another advantageous embodiment, a liquid is introducedinto the hole delimiting the via as a metal-containing substance as analternative or in addition to the previously described powderysubstance, by the ceramic layer being sprayed with the liquid orimmersed in the liquid, wherein a subsequent, additional thermalconversion step for converting the liquid into copper oxide, e.g.copper(I) oxide and/or copper(II) oxide, is carried out in a temperaturerange of approximately 100° C. to approximately 200° C. prior to thehigh-temperature step. The liquid is advantageous in that it is able topenetrate even holes with a very small diameter and adheres to theceramic layer in the ceramic holes due to the surface tension. In thecase in which one of the two metal layers is already attached to one ofthe two surface sides of the ceramic layer and the via hole is thusclosed on one side, a liquid column filling the hole can advantageouslyform in the hole.

Particularly preferably, a copper(II) hydroxide brine or copper(II)acetate brine is used as the metal-containing liquid.

The two metal layers can be attached to the ceramic layer by means ofthe DCB method (direct copper bonding) known per se, wherein the powderyor liquid metal-containing substance can be composed as described above.

Alternatively, the two metal layers can also be attached to the ceramiclayer by means of the AMB method (active metal brazing), which is alsoknown. In this case, the AMB solder can advantageously also be useddirectly as the metal-containing substance.

Another embodiment of the invention, which is particularly advantageouswith respect to process efficiency, provides that one of the two metallayers is attached to the ceramic layer simultaneously with thehigh-temperature step for wetting the ceramic layer with themetal-containing substance.

After attaching the two metal layers to the ceramic layer, themetal-ceramic substrate can be subjected to hot isostatic pressing. Aninternal porosity of the at least one via can thus be eliminatedsubsequently, and the electrical conductivity of the via can be improvedor ensured.

In order to eliminate uneven portions on the external surfaces of thetwo metal layers, particularly on the hole delimiting the via, theceramic substrate metallized on both sides can be ground on its externalmetal surfaces.

According to yet another advantageous embodiment of the invention,aluminum oxide (Al₂O₃) is admixed as a ceramic filler to themetal-containing substance. The coefficient of thermal expansion of aeutectic intermediate layer between the metal of the substance and theceramic layer is adjusted by the content of the ceramic material in themetal-containing substance, so that fewer mechanical stresses ariseduring thermal cycles. This results in a higher reliability or longerlife of the material composite produced, and thus of the electricalconductivity of the via.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention become apparent from thefollowing description as well of exemplary embodiments of the invention,which shall be understood not to be limiting and which will be explainedbelow with reference to the drawing. In this drawing, the Figuresschematically show:

FIG. 1 illustrates four process steps (a-d) of a first exemplaryembodiment of a method according to the invention;

FIG. 2 illustrates four process steps (a-d) of a second exemplaryembodiment of a method according to the invention; and

FIG. 3 illustrates a metal-ceramic substrate produced with the method ofFIG. 2 in a lateral sectional view (a) and a top view (b) sectionedalong the section line A-A depicted in the lateral sectional view (a).

DETAILED DESCRIPTION

In the different figures, parts that are equivalent with respect totheir function are always provided with the same reference numerals, sothat they are also only described once, as a rule.

FIG. 1 illustrates four process steps (a-d) of a first exemplaryembodiment of a method according to the invention for the production ofa metal-ceramic substrate 1 metallized on both sides with anelectrically conductive via 2 shown in an exemplary manner in the fourdetailed views a) to d). In its entirety, the metal-ceramic substrate 1has a plurality of the vias 2 shown in view d).

As is apparent from FIG. 1, the metal-ceramic substrate 1 has a ceramicplate or ceramic layer 3, such as Al₂O₃ or AlN ceramics. On the lowersurface side of the ceramic layer 3 shown in FIG. 1, a first metal layer4 has already been attached in a planar manner to the ceramic layer 3prior to the process step a), for example a copper layer by means of theknown DCB method. The ceramic layer 3 further has a hole 5 delimitingthe electrical via 2. The hole 5 preferably has a diameter ofapproximately 50 μm to approximately 2000 μm. For example, it may beintroduced into the ceramic layer 3 by lasering after sintering theceramic layer 3, or by punching the green compact in a correspondingmanner prior to sintering.

In the process step b) shown in FIG. 1, a metal-containing powderysubstance 6 was introduced into the hole 5. In the case shown here, themetal-containing substance 6 is a powder mixture composed of copper (Cu)and additionally at least one element from the group consisting ofcopper(I) oxide (Cu₂O), copper(II) oxide (CuO) and copper(II) hydroxide(Cu(OH)₂), wherein the content of copper in the powder mixture isbetween 0% and approximately 95% and the at least one element from theaforementioned group forms the rest up to 100%.

Then, this assembly consisting of the ceramic layer 3, the metal layer 4attached to a surface side of the ceramic layer 3 and the powder mixture6 is subjected to a high-temperature step above 500° C., e.g. to aconventional DCB high-temperature step, in which the metal-containingsubstance 6 wets the inner wall of the hole 5 at least partially with awetting angle of less than 90°, so that a material bond is formedbetween the ceramic layer 3 and the metal of the powder mixture 6.Process step c) in FIG. 1 depicts the state after this high-temperaturestep, in which the powder mixture 6 has reacted to form a (porous)copper body 7.

In a further high-temperature step, e.g. a DCB high-temperature step, asecond metal layer 8 is then attached in a conventional manner to asurface side of the ceramic layer 3 opposing the first metal layer 4. Inthe process, the copper body 7 produces an electrically conductiveconnection between the two metal layers 4 and 8, as is shown in processstep d) of FIG. 1.

FIG. 2 illustrates four process steps (a-d) of a second exemplaryembodiment of a method according to the invention for the production ofa metal-ceramic substrate 10 metallized on both sides with anelectrically conductive via 11 shown in an exemplary manner in the fourdetailed views a) to d). In its entirety, the metal-ceramic substrate 10has a plurality of the vias 11 shown in view d).

As is apparent from FIG. 2, the metal-ceramic substrate 10 has a ceramicplate or ceramic layer 3, such as Al₂O₃ or AlN ceramics. On the lowersurface side of the ceramic layer 3 shown in FIG. 2, a first metal layer4 has already been attached in a planar manner to the ceramic layer 3prior to the process step a), for example a copper layer by means of theknown DCB method.

As is also apparent from process step a) of FIG. 2, the ceramic layer 3has a hole 12 delimiting the electrical via 11, which in the case shownhere has a considerably smaller diameter than the hole 5 shown in FIG.1.

The main difference between the method shown in FIG. 2 and the methodshown in FIG. 1 is that the hole 11 of the ceramic layer 3 in processstep b) of FIG. 2 was treated with brine 13 instead of with ametal-containing powder mixture 6, in particular with a copper hydroxideor copper acetate brine 13, for example by spraying, immersing theceramic layer 3 into the brine 13 or the like. Due to the surfacetension, the brine 13 is drawn into the hole 12, as is shown in processstep b) of FIG. 2.

Then, as shown in process step c) of FIG. 2, an additional thermalconversion step for converting the brine 13 into copper oxide 14 iscarried out in a temperature range of approximately 100° C. toapproximately 200° C. prior to the high-temperature step (not shown inFIG. 2) for forming a copper body 7 in the hole 12 from the copper oxide14.

In a further high-temperature step, e.g. a DCB high-temperature step, asecond metal layer 8 is finally attached in a conventional manner to asurface side of the ceramic layer 3 opposing the first metal layer 4. Inthe process, the copper body 7 produces an electrically conductiveconnection between the two metal layers 4 and 8, as is shown in processstep d) of FIG. 2.

FIG. 3 illustrates the metal-ceramic substrate 10 produced with themethod of FIG. 2 in a lateral sectional view (a) and a top view (b)sectioned along the section line A-A depicted in the lateral sectionalview (a).

As is apparent from view (a) of FIG. 3, a semiconductor component 15 isattached to the upper metal layer 8. A plurality of vias 11, each withrelatively small diameters, is disposed in a clustered manner underneaththe semiconductor component 15. The clustering of the vias 11 makes itpossible to ensure the same current-carrying capacity per unit area. Theheat dissipation from the semiconductor component 15 can also beimproved thereby. Preferably, the vias 11 are therefore disposedunderneath the semiconductor component 15, whose outline is shown inview (b) of FIG. 3 as an aid for better illustration.

The above-described inventive method for producing a metal-ceramicsubstrate with at least one electrically conductive via is not limitedto the embodiments disclosed herein, but also includes embodimentshaving the same effects. For example, it is conceivable directly to use,instead of the liquid sole shown in FIG. 2, a liquid AMB solder if themetallization of the ceramic layer is carried out by means of the AMBmethod known per se, for example.

Furthermore, the above-described invention can in principle be appliedto any type of ceramic substrate, for example AlN (aluminum nitride),Si₃N₄ (silicon nitride), Al₂O₃ (aluminum oxide) and the like, which canbe coated with a metal layer, e.g. Cu (copper) or Al (aluminum) or analloy thereof. In the process, the metallization can be applied to twoopposing surface sides of the substrate by means of different methods,e.g. by AMB (active metal brazing), DCB (direct copper bonding), DAB(direct aluminum bonding), thick-film methods and the like. DCB and AMBceramic substrates are particularly preferred. Here, the term“substrate” is used as a synonym for all of the above-mentioned types ofsubstrate.

In a preferred embodiment, the metal-ceramic substrate produced by meansof the method according to the invention is used for the fabrication ofelectric circuits, particularly of power circuits.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

LIST OF REFERENCE NUMERALS

-   1 Metal-ceramic substrate-   2 Via-   3 Ceramic layer-   4 First metal layer-   5 Hole-   6 Powdery metal-containing substance-   7 Copper body-   8 Second metal layer-   10 Metal-ceramic substrate-   11 Via-   12 Hole-   13 Brine-   14 Brine converted into copper oxide-   15 Semiconductor component

What is claimed is:
 1. A method for producing a metal-ceramic substratewith at least one electrically conductive via, the method comprising:attaching a first metal layer in a planar manner to a first surface sideof a ceramic layer and attaching a second metal layer in a planar mannerto a second surface side of the ceramic layer opposite the first surfaceside; introducing a metal-containing powdery substance into a hole inthe ceramic layer delimiting the at least one electrically conductivevia prior to the attachment of both the first and the second metallayers, or subsequent to the attachment of one of the first and thesecond metal layers and prior to the attachment of the other one of thefirst and the second metal layers, to form an assembly; and subjectingthe assembly to a high-temperature step above 500° C. in which themetal-containing powdery substance wets the ceramic layer at leastpartially with a wetting angle of less than 90°.
 2. The method of claim1, wherein the first metal layer is attached to the first surface sideof the ceramic layer prior to the introduction of the metal-containingpowdery substance into the hole delimiting the at least one electricallyconductive via.
 3. The method of claim 1, wherein the metal-containingpowdery substance is a powder mixture comprising copper and at least onecompound selected from the group consisting of copper(I) oxide,copper(II) oxide and copper(II) hydroxide.
 4. The method of claim 3,wherein grain sizes of the powder mixture are at most 90% of thediameter of the hole delimiting the at least one electrically conductivevia.
 5. The method of claim 1, wherein the first and the second metallayers are attached to the ceramic layer by means of a direct copperbonding (DCB) method.
 6. The method of claim 1, wherein the first andthe second metal layers are attached to the ceramic layer by means of anactive metal brazing (AMB) method, and wherein the metal-containingpowdery substance is an AMB solder.
 7. The method of claim 1, whereinone of the first and the second metal layers is attached to the ceramiclayer simultaneously with the high-temperature step for wetting theceramic layer with the metal-containing powdery substance.
 8. The methodof claim 1, further comprising: subjecting the metal-ceramic substrateto hot isostatic pressing after attaching the first and the second metallayers to the ceramic layer.
 9. The method of claim 1, furthercomprising: grinding the metal-ceramic substrate on opposing externalmetal surfaces after attaching the first and the second metal layers tothe ceramic layer.
 10. The method of claim 1, wherein aluminum oxide isadmixed as a ceramic filler to the metal-containing powdery substance.